bundle2: seek part back during iteration

Previously, iterparts would yield the part to users, then consume the part. This
changed the part after the user was given it and left it at the end, both of
which seem unexpected.  Let's seek back to the beginning after we've consumed
it. I tried not seeking to the end at all, but that seems important for the
overall bundle2 consumption.

This is used in a future patch to let us move the bundlerepo
bundle2-changegroup-part to be handled entirely within the for loop, instead of
having to do a seek back to 0 after the entire loop finishes.

Differential Revision: https://phab.mercurial-scm.org/D289

mercurial/bundle2.py

@@ -828,7 +828,11 b' class unbundle20(unpackermixin):'
         while headerblock is not None:
             part = unbundlepart(self.ui, headerblock, self._fp)
             yield part
+            # Seek to the end of the part to force it's consumption so the next
+            # part can be read. But then seek back to the beginning so the
+            # code consuming this generator has a part that starts at 0.
             part.seek(0, 2)
+            part.seek(0)
             headerblock = self._readpartheader()
         indebug(self.ui, 'end of bundle2 stream')
 

mercurial/bundlerepo.py

@@ -301,7 +301,6 b' class bundlerepository(localrepo.localre'
 
             if cgstream is None:
                 raise error.Abort(_('No changegroups found'))
-            cgstream.seek(0)
 
             self.bundle = changegroup.getunbundler(version, cgstream, 'UN')